Aircraft having an aft engine

ABSTRACT

An aircraft is provided including a fuselage that extends along a longitudinal direction between a forward end and an aft end. A boundary layer ingestion fan is mounted to the fuselage at the aft end and is configured for ingesting boundary layer airflow off the surface of the fuselage. The fuselage defines a profile proximate the boundary layer ingestion fan that is optimized for ingesting a maximum amount of boundary layer air and improving propulsive efficiency of the aircraft. More specifically, the fuselage defines a cross sectional profile upstream of the boundary layer ingestion fan that has more cross sectional area in a top half relative to a bottom half as defined relative to a centerline of the boundary layer ingestion fan.

FIELD OF THE INVENTION

The present subject matter relates generally to an aircraft having anaft engine, or more particularly to a fuselage of an aircraft designedto increase the efficiency of the aft engine.

BACKGROUND OF THE INVENTION

A conventional commercial aircraft generally includes a fuselage, a pairof wings, and a propulsion system that provides thrust. The propulsionsystem typically includes at least two aircraft engines, such asturbofan jet engines. Each turbofan jet engine is mounted to arespective one of the wings of the aircraft, such as in a suspendedposition beneath the wing, separated from the wing and fuselage. Such aconfiguration allows for the turbofan jet engines to interact withseparate, freestream airflows that are not impacted by the wings and/orfuselage. This configuration can reduce an amount of turbulence withinthe air entering an inlet of each respective turbofan jet engine, whichhas a positive effect on a net propulsive thrust of the aircraft.

However, a drag on the aircraft including the turbofan jet engines alsoaffects the net propulsive thrust of the aircraft. A total amount ofdrag on the aircraft, including skin friction and form drag, isgenerally proportional to a difference between a freestream velocity ofair approaching the aircraft and an average velocity of a wakedownstream from the aircraft that is produced due to the drag on theaircraft.

Positioning a fan at an aft end of the fuselage of the aircraft mayassist with reenergizing a boundary layer airflow over the aft end ofthe fuselage and improving propulsive efficiency. However, givenexisting structures at the aft end of the fuselage, such as one or morestabilizers, the airflow ingested by such a fan may not have a uniformvelocity or total pressure profile along the circumferential and radialdirections of the fan. More specifically, the structures at the aft endof the fuselage may generate a boundary layer or wake resulting in swirldistortion and a distorted velocity or total pressure profile of theairflow ingested by the fan.

Accordingly, an aircraft capable of energizing slow-moving air forming aboundary layer across the fuselage of the aircraft would be useful.Specifically, a fuselage of an aircraft designed to increase theingestion of relatively low momentum boundary layer airflow into the aftengine and reduce the non-uniformity and distortion of the velocityprofile of ingested airflow would be particularly beneficial.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one exemplary embodiment of the present disclosure, an aircraftdefining a longitudinal direction, a vertical direction, and a lateraldirection is provided. The aircraft includes a fuselage extendingbetween a forward end and an aft end along the longitudinal direction.The aircraft also includes a boundary layer ingestion fan mounted to thefuselage at the aft end of the fuselage, the boundary layer ingestionfan defining a centerline and including a plurality of fan bladesrotatable about the centerline and a nacelle surrounding the pluralityof fan blades. The fuselage defines a cross section upstream of theboundary layer ingestion fan, the cross section defining a horizontalreference line extending through the centerline of the boundary layeringestion fan to define a top half having a top half cross sectionalarea and a bottom half having a bottom half cross sectional area. Thetop half cross sectional area of the cross section is greater than thebottom half cross sectional area of the cross section.

In another exemplary embodiment of the present disclosure, an aircraftdefining a longitudinal direction, a vertical direction, and a lateraldirection is provided. The aircraft includes a fuselage extendingbetween a forward end and an aft end along the longitudinal direction,the fuselage defining a top surface and a bottom surface. The aircraftalso includes a boundary layer ingestion fan mounted to the fuselage atthe aft end of the fuselage, the boundary layer ingestion fan defining acenterline and including a plurality of fan blades rotatable about thecenterline and a nacelle surrounding the plurality of fan blades. Thefuselage defines a cross section upstream of the boundary layeringestion fan, the cross section defining a circumference and ahorizontal reference line, wherein the horizontal reference line extendsacross a widest portion of the cross section along the lateraldirection, the fuselage further defining a reference circle at the crosssection and having the horizontal reference line as a diameter of thereference circle. At least a portion of the circumference of a top halfof the cross section of the fuselage is located outside the referencecircle, and wherein at least a portion of the circumference of a bottomhalf of the cross section of the fuselage is located inside thereference circle.

In yet another exemplary embodiment of the present disclosure, anaircraft defining a longitudinal direction, a vertical direction, and alateral direction is provided. The aircraft includes a fuselageextending between a forward end and an aft end along the longitudinaldirection, the fuselage defining a surface. A boundary layer ingestionfan is mounted to the fuselage at the aft end of the fuselage, theboundary layer ingestion fan defining a centerline and including aplurality of fan blades rotatable about the centerline and a nacellesurrounding the plurality of fan blades. A stabilizer is attached to thefuselage and extending between a leading edge and a trailing edge. Thesurface of the fuselage defines a first point located in a planeperpendicular to the longitudinal direction and positioned where theleading edge of the stabilizer meets the fuselage, a second pointlocated in a plane perpendicular to the longitudinal direction andpositioned where the trailing edge of the stabilizer meets the fuselage,and an inflection point. The surface of the fuselage further defines afirst portion of the surface, the first portion extending between thefirst point and the inflection point, the first portion being convex;and a second portion of the surface, the second portion extendingbetween the inflection point and the second point, the second portionbeing concave.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures.

FIG. 1 is a top view of an aircraft according to various exemplaryembodiments of the present disclosure.

FIG. 2 is a port side view of the exemplary aircraft of FIG. 1.

FIG. 3 is a schematic, cross-sectional view of a gas turbine engine inaccordance with an exemplary embodiment of the present disclosure.

FIG. 4 is a schematic, cross-sectional view of an aft engine mounted toan aft end of the exemplary aircraft of FIG. 1 in accordance with anexemplary embodiment of the present disclosure.

FIG. 5 provides another schematic, cross-sectional side view of an aftengine mounted to an aft end of the exemplary aircraft of FIG. 1 inaccordance with an exemplary embodiment of the present disclosure.

FIG. 6 provides another schematic, cross-sectional side view of an aftengine mounted to an aft end of the exemplary aircraft of FIG. 1 inaccordance with an exemplary embodiment of the present disclosure.

FIG. 7 provides a schematic cross-sectional view of the fuselage of theexemplary aircraft of FIG. 1, as taken along Line X-X of FIG. 5according to an exemplary embodiment of the present subject matter.

FIG. 8 provides a schematic cross-sectional view of the fuselage of theexemplary aircraft of FIG. 1, as taken along Line X-X of FIG. 5according to another exemplary embodiment of the present subject matter.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to present embodiments of theinvention, one or more examples of which are illustrated in theaccompanying drawings. The detailed description uses numerical andletter designations to refer to features in the drawings. Like orsimilar designations in the drawings and description have been used torefer to like or similar parts of the invention. As used herein, theterms “first”, “second”, and “third” may be used interchangeably todistinguish one component from another and are not intended to signifylocation or importance of the individual components. The terms“upstream” and “downstream” refer to the relative direction with respectto fluid flow in a fluid pathway. For example, “upstream” refers to thedirection from which the fluid flows, and “downstream” refers to thedirection to which the fluid flows.

Referring now to the drawings, wherein identical numerals indicate thesame elements throughout the figures, FIG. 1 provides a top view of anexemplary aircraft 10 as may incorporate various embodiments of thepresent invention. FIG. 2 provides a port side 24 view of the aircraft10 as illustrated in FIG. 1. As shown in FIGS. 1 and 2 collectively, theaircraft 10 defines a longitudinal direction 12 that extendstherethrough, a vertical direction V, a lateral direction L, a forwardend 14, and an aft end 16. Moreover, the aircraft 10 defines a mean line18 extending between the forward end 14 and aft end 16 of the aircraft10. As used herein, the “mean line” refers to a midpoint line extendingalong a length of the aircraft 10, not taking into account theappendages of the aircraft 10 (such as the wings 22 and stabilizersdiscussed below).

Moreover, the aircraft 10 includes a fuselage 20, extendinglongitudinally from the forward end 14 of the aircraft 10 towards theaft end 16 of the aircraft 10, and a pair of wings 22. As used herein,the term “fuselage” generally includes all of the body of the aircraft10, such as an empennage of the aircraft 10 and an outer surface or skinof the aircraft 10. The first of such wings 22 extends laterallyoutwardly with respect to the longitudinal direction 12 from the portside 24 of the fuselage 20 and the second of such wings 22 extendslaterally outwardly with respect to the longitudinal direction 12 from astarboard side 26 of the fuselage 20. Each of the wings 22 for theexemplary embodiment depicted includes one or more leading edge flaps 28and one or more trailing edge flaps 30. The aircraft 10 further includesa vertical stabilizer 32 having a rudder flap 34 for yaw control, and apair of horizontal stabilizers 36, each having an elevator flap 38 forpitch control. The fuselage 20 additionally includes an outer surface40.

As illustrated, each stabilizer extends between a root portion and a tipportion substantially within a single plane. For example, as illustratedin FIGS. 1 and 2, vertical stabilizer 32 defines a root portion 60 and atip portion 62 separated along the vertical direction V. In addition,vertical stabilizer 32 extends between a leading edge 64 and a trailingedge 66 along the longitudinal direction 12. As illustrated, verticalstabilizer 32 is mounted to fuselage 20 at root portion 60 and extendssubstantially along the vertical direction V to tip portion 62. In thismanner, a junction line 68 is defined at the intersection of verticalstabilizer 32 and fuselage 20. More specifically, junction line 68extends between leading edge 64 and trailing edge 66 of verticalstabilizer 32. However, it should be appreciated that in other exemplaryembodiments of the present disclosure, the aircraft 10 may additionallyor alternatively include any other suitable configuration of stabilizersthat may or may not extend directly along the vertical direction V orhorizontal/lateral direction L. In addition, alternative stabilizers maybe any suitable shape, size, configuration, or orientation whileremaining within the scope of the present subject matter.

The exemplary aircraft 10 of FIGS. 1 and 2 also includes a propulsionsystem. The exemplary propulsion system includes a plurality of aircraftengines, at least one of which mounted to each of the pair of wings 22.Specifically, the plurality of aircraft engines includes a firstaircraft engine 42 mounted to a first wing of the pair of wings 22 and asecond aircraft engine 44 mounted to a second wing of the pair of wings22. In at least certain exemplary embodiments, the aircraft engines 42,44 may be configured as turbofan jet engines suspended beneath the wings22 in an under-wing configuration. For example, in at least certainexemplary embodiments, the first and/or second aircraft engines 42, 44may be configured in substantially the same manner as the exemplaryturbofan jet engine 100 described below with reference to FIG. 3.Alternatively, however, in other exemplary embodiments any othersuitable aircraft engine may be provided. For example, in otherexemplary embodiments the first and/or second aircraft engines 42, 44may alternatively be configured as turbojet engines, turboshaft engines,turboprop engines, etc.

Additionally, the propulsion system includes an aft engine 200 mountedto the fuselage 20 of the aircraft 10 proximate the aft end 16 of theaircraft 10, or more particularly at a location aft of the wings 22 andaircraft engines 42, 44. The exemplary aft engine 200 is mounted to thefuselage 20 of the aircraft 10 such that the mean line 18 extendstherethrough. The aft engine 200, which is generally configured as anengine that ingests and consumes air forming a boundary layer overfuselage 20, will be discussed in greater detail below with reference toFIGS. 4 through 8.

Referring specifically to FIG. 2, the aircraft 10 additionally includeslanding gear, such as wheels 46, extending from a bottom side of thefuselage 20 and from a bottom side of the wings 22. The fuselage 20 isdesigned to allow the aircraft 10 to takeoff and/or land at a takeoffangle 48 with the ground without the aft end 16 scraping the ground.More specifically, takeoff angle 48 may be defined as the angle betweenthe ground (parallel to longitudinal direction 12) and a takeoff plane50. As will be discussed below, the exemplary fuselage 20 and aft engine200 described herein are designed to allow the aircraft 10 to maintain adesired takeoff angle 48, despite the addition of the aft engine 200proximate the aft end 16 of the aircraft 10. Notably, for the embodimentdepicted, the longitudinal direction 12 of the aircraft 10 is parallelto the ground when the aircraft 10 is on the ground. Accordingly, themaximum takeoff angle 48, as shown, may alternatively be defined withthe longitudinal direction 12 of the aircraft 10 (shown as angle 48′ inFIG. 2).

Referring now to FIG. 3, a schematic, cross-sectional view of anexemplary aircraft engine is provided. Specifically, for the embodimentdepicted, the aircraft engine is configured as a high bypass turbofanjet engine, referred to herein as “turbofan engine 100.” As discussedabove, one or both of the first and/or second aircraft engines 42, 44 ofthe exemplary aircraft 10 described in FIGS. 1 and 2 may be configuredin substantially the same manner as the exemplary turbofan engine 100 ofFIG. 3. Alternatively, however, in other exemplary embodiments, one orboth of aircraft engines 42, 44 may be configured as any other suitableengines, such as a turboshaft, turboprop, turbojet, etc.

As shown in FIG. 3, the turbofan engine 100 defines an axial directionA₁ (extending parallel to a longitudinal centerline 102 provided forreference) and a radial direction R₁. In general, the turbofan 10includes a fan section 104 and a core turbine engine 106 disposeddownstream from the fan section 104.

The exemplary core turbine engine 106 depicted generally includes asubstantially tubular outer casing 108 that defines an annular inlet110. The outer casing 108 encases, in serial flow relationship, acompressor section including a booster or low pressure (LP) compressor112 and a high pressure (HP) compressor 114; a combustion section 116; aturbine section including a high pressure (HP) turbine 118 and a lowpressure (LP) turbine 120; and a jet exhaust nozzle section 122. A highpressure (HP) shaft or spool 124 drivingly connects the HP turbine 118to the HP compressor 114. A low pressure (LP) shaft or spool 126drivingly connects the LP turbine 120 to the LP compressor 112. Thecompressor section, combustion section 116, turbine section, and nozzlesection 122 together define a core air flowpath.

For the embodiment depicted, the fan section 104 includes a variablepitch fan 128 having a plurality of fan blades 130 coupled to a disk 132in a spaced apart manner. As depicted, the fan blades 130 extendoutwardly from disk 132 generally along the radial direction R₁ anddefine a fan diameter D. Each fan blade 130 is rotatable relative to thedisk 132 about a pitch axis P by virtue of the fan blades 130 beingoperatively coupled to a suitable actuation member 134 configured tocollectively vary the pitch of the fan blades 130 in unison. The fanblades 130, disk 132, and actuation member 134 are together rotatableabout the longitudinal direction 12 by LP shaft 126 across a power gearbox 136. The power gear box 136 includes a plurality of gears foradjusting the rotational speed of the fan 128 relative to the LP shaft126 to a more efficient rotational fan speed.

Referring still to the exemplary embodiment of FIG. 3, the disk 132 iscovered by rotatable front hub 138 aerodynamically contoured to promotean airflow through the plurality of fan blades 130. Additionally, theexemplary fan section 104 includes an annular fan casing or outernacelle 140 that circumferentially surrounds the fan 128 and/or at leasta portion of the core turbine engine 106. It should be appreciated thatthe nacelle 140 may be configured to be supported relative to the coreturbine engine 106 by a plurality of circumferentially-spaced outletguide vanes 142. Moreover, a downstream section 144 of the nacelle 140may extend over an outer portion of the core turbine engine 106 so as todefine a bypass airflow passage 146 therebetween.

It should be appreciated, however, that the exemplary turbofan engine100 depicted in FIG. 3 is by way of example only, and that in otherexemplary embodiments, the turbofan engine 100 may have any othersuitable configuration, including, e.g., any suitable number of shaftsor spools, compressors, and/or turbines.

Referring now also to FIG. 4, a close-up, schematic, cross-sectionalview of the exemplary aft engine 200 of FIGS. 1 and 2 is provided. Asdiscussed, the exemplary aft engine 200 is mounted to the fuselage 20proximate the aft end 16 of the aircraft 10. The aft engine 200 depicteddefines an axial direction A₂ extending along a longitudinal centerlineaxis 220 that extends therethrough for reference, a radial direction R₂,and a circumferential direction C₂ (see FIG. 7).

Additionally, for the embodiment depicted, the aft engine 200 isconfigured as a boundary layer ingestion engine configured to ingest andconsume air forming a boundary layer over the fuselage 20 of theaircraft 10. The aft engine 200 includes a fan 222 rotatable about thecenterline axis 220, a nacelle 224 extending around a portion of the fan222, and one or more structural members 226 extending between thenacelle 224 and the fuselage 20 of the aircraft 10. The fan 222 includesa plurality of fan blades 228 spaced generally along circumferentialdirection C₂. Additionally, the nacelle 224 extends around and encirclesthe plurality of fan blades 228 and a portion of the fuselage 20.Specifically, the nacelle 224 extends around at least a portion of thefuselage 20 of the aircraft 10 when, as in FIG. 4, the aft engine 200 ismounted to the aircraft 10.

As is also depicted in FIG. 4, the fan 222 further includes a fan shaft230 with the plurality of fan blades 228 attached thereto. Although notdepicted, the fan shaft 230 may be rotatably supported by one or morebearings located forward of the plurality of fan blades 228 and,optionally, one or more bearings located aft of the plurality of fanblades 228. Such bearings may be any suitable combination of rollerbearings, ball bearings, thrust bearings, etc.

In certain exemplary embodiments, the plurality of fan blades 228 may beattached in a fixed manner to the fan shaft 230, or alternatively, theplurality of fan blades 228 may be rotatably attached to the fan shaft230. For example, the plurality of fan blades 228 may be attached to thefan shaft 230 such that a pitch of each of the plurality of fan blades228 may be changed, e.g., in unison, by a pitch change mechanism (notshown).

The fan shaft 230 is mechanically coupled to a power source 232 locatedat least partially within the fuselage 20 of the aircraft 10. For theembodiment depicted, the fan shaft 230 is mechanically coupled to thepower source 232 through a gearbox 234. The gearbox 234 may beconfigured to modify a rotational speed of the power source 232, orrather of a shaft 236 of the power source 232, such that the fan 222 ofthe aft engine 200 rotates at a desired rotational speed. The gearbox234 may be a fixed ratio gearbox, or alternatively, the gearbox 234 maydefine a variable gear ratio.

The power source 232 may be any suitable power source. For example, incertain exemplary embodiments the power source 232 may be an electricpower source (e.g., the aft engine 200 may be configured as part of agas-electric propulsion system with the first and/or second aircraftengines 42, 44). However, in other exemplary embodiments, the powersource 232 may alternatively be configured as a dedicated gas engine,such as a gas turbine engine. Moreover, in certain exemplaryembodiments, the power source 232 may be positioned at any othersuitable location within, e.g., the fuselage 20 of the aircraft 10 orthe aft engine 200. For example, in certain exemplary embodiments, thepower source 232 may be configured as a gas turbine engine positioned atleast partially within the aft engine 200.

Referring still to FIG. 4, the one or more structural members 226 extendbetween the nacelle 224 and the fuselage 20 of the aircraft 10 at alocation forward of the plurality of fan blades 228. The one or morestructural members 226 for the embodiment depicted extend substantiallyalong the radial direction R₂ between the nacelle 224 and the fuselage20 of the aircraft 10 for mounting the aft engine 200 to the fuselage 20of the aircraft 10. It should also be appreciated, however, that inother exemplary embodiments the one or more structural members 226 mayinstead extend substantially along the axial direction A₂, or in anyother suitable direction between the axial and radial directions A₂, R₂.It should be appreciated, that as used herein, terms of approximation,such as “approximately,” “substantially,” or “about,” refer to beingwithin a ten percent margin of error.

The one or more structural members 226 depicted are configured as inletguide vanes for the fan 222, such that the one or more structuralmembers 226 are shaped and oriented to direct and condition a flow ofair into the aft engine 200 to increase an efficiency of the aft engine200. In certain exemplary embodiments, the one or more structuralmembers 226 may be configured as fixed inlet guide vanes extendingbetween the nacelle 224 and the fuselage 20 of the aircraft 10, oralternatively the one or more structural members 226 may be configuredas variable inlet guide vanes.

Moreover, the aft engine 200 includes one or more outlet guide vanes 238and a tail cone 240. The one or more outlet guide vanes 238 for theembodiment depicted extend between the nacelle 224 and the tail cone 240for, e.g., adding strength and rigidity to the aft engine 200, as wellas for directing a flow of air through the aft engine 200. The outletguide vanes 238 may be evenly spaced along the circumferential directionC₂ (see FIG. 7), or may have any other suitable spacing. Additionally,the outlet guide vanes 238 may be fixed outlet guide vanes, oralternatively may be variable outlet guide vanes.

Aft of the plurality of fan blades 228, and for the embodiment depicted,aft of the one or more outlet guide vanes 238, the aft engine 200additionally defines a nozzle 242 between the nacelle 224 and the tailcone 240. The nozzle 242 may be configured to generate an amount ofthrust from the air flowing therethrough, and the tail cone 240 may beshaped to minimize an amount of drag on the aft engine 200. However, inother embodiments, the tail cone 240 may have any other shape and may,e.g., end forward of an aft end of the nacelle 224 such that the tailcone 240 is enclosed by the nacelle 224 at an aft end. Additionally, inother embodiments, the aft engine 200 may not be configured to generateany measurable amount of thrust, and instead may be configured to ingestair from a boundary layer of air of the fuselage 20 of the aircraft 10and add energy/speed up such air to reduce an overall drag on theaircraft 10 (and thus increase a net thrust of the aircraft 10).

Referring still to FIG. 4, the aft engine 200, or rather the nacelle224, defines an inlet 244 at a forward end 246 of the nacelle 224. Theinlet 244 is defined by the nacelle 224 with the fuselage 20, i.e.,between the nacelle 224 and the fuselage 20. As mentioned above, thenacelle 224 of the aft engine 200 extends around and surrounds theplurality of fan blades 228 of the fan 222 of the aft engine 200. Forthe embodiment depicted, nacelle 224 also extends at least partiallyaround the central axis 220 of the aft engine 200, and at leastpartially around the mean line 18 of the aircraft 10. Specifically, forthe embodiment depicted, the nacelle 224 extends substantially threehundred and sixty degrees (360°) around the central axis 220 of the aftengine 200, and substantially three hundred and sixty degrees (360°)around the mean line 18 of the aircraft 10.

Notably, by positioning the aft engine 200 such that the nacelle 224 ofthe aft engine 200 extends at least partially around the fuselage 20proximate the aft end 16 of the aircraft 10, a bottom portion 248 of thenacelle 224 may not interfere with, e.g., the takeoff angle 48 of theaircraft 10 (see FIG. 2). For example, as shown, the nacelle 224 of theaft engine 200 includes at least a portion located inward of the takeoffplane 50 defined by the fuselage 20 (see FIG. 2). Particularly for theembodiment depicted, an entirety of the bottom portion 248 of thenacelle 224 is positioned in-line with, or inwardly of the takeoff plane50 of the fuselage 20. For at least certain prior art aircrafts, thetakeoff plane 50 of the fuselage 20 indicates the conventional shape fora bottom portion of a fuselage at an aft end of an aircraft.

Referring now to FIGS. 5 through 8, the shape of the aft end 16 of theexemplary aircraft 10 as well as features for providing improvedboundary layer ingestion will be described in more detail. Morespecifically, FIGS. 5 and 6 provide schematic, cross-sectional sideviews of aft engine 200 mounted to fuselage 20. FIGS. 7 and 8 provideschematic cross-sectional views of fuselage, taken along the Line X-X inFIG. 5.

Referring specifically to FIG. 5, according to an exemplary embodiment,top side 202 of fuselage 20 defines a top surface 302 along whichboundary layer air flows over aircraft 10. Similarly, bottom side 204defines a bottom surface 304 along which boundary layer air flows overaircraft 10. As explained above, it is desirable to accelerate lowvelocity boundary layer airflow to improve propulsive efficiency. Thefeatures of the aircraft 10 described herein achieve these and otherobjectives.

According to the illustrated embodiment, top surface 302 defines a firstpoint 310 located in a plane perpendicular to the longitudinal direction12 and positioned at or aft of where leading edge 64 of verticalstabilizer 32 meets fuselage 20. In addition, top surface 302 defines asecond point 312 located in a plane perpendicular to the longitudinaldirection 12 downstream of first point 310. For example, second point312 may be positioned at or forward of where trailing edge 66 ofvertical stabilizer 32 meets fuselage 20. Top surface 302 also definesan upper inflection point 314 positioned between first point 310 andsecond point 312 along top surface 302 of fuselage 20. As illustrated, afirst portion 316 of top surface 302 extends between first point 310 andupper inflection point 314 and a second portion 318 of top surface 302extends between upper inflection point 314 and second point 312.

As illustrated in FIG. 5, first portion 316 is a convex curve whenviewed looking down onto top surface 302 from outside of fuselage 20. Inaddition, second portion 318 is a concave curve when viewed looking downonto top surface 302 from outside of fuselage 20. In this regard,fuselage 20 generally defines a convex surface upstream a concavesurface proximate aft engine 200. In this manner, boundary layer airflowmay more effectively be distributed within aft engine 200. Thus, aftengine 200 may ingest a desired amount of slower moving boundary layerairflow and may discharge that low velocity air as relatively highervelocity air, thereby improving the propulsive efficiency of aircraft10.

It should be appreciated that bottom surface 304 and any other surfacelocated circumferentially around fuselage 20 proximate aft end 16 offuselage 20 may have a similar profile as top surface 302. For example,bottom surface 304 defines a first point 320 located, for example, inthe same plane as first point 310. In addition, bottom surface 304defines a second point 322 located, for example, in the same plane assecond point 312. It should be appreciated that first point 320 andsecond point 322 may alternatively be positioned at any suitablelocation along bottom surface 304 of fuselage 20. Bottom surface 304also defines a lower inflection point 324 positioned between first point320 and second point 322 along bottom surface 304 of fuselage 20. Asillustrated, a convex first portion 326 of bottom surface 304 extendsbetween first point 320 and lower inflection point 324 and a concavesecond portion 328 of bottom surface 304 extends between lowerinflection point 324 and second point 322.

As illustrated, fuselage 20 defines upper inflection point 314 and lowerinflection point 324 upstream of inlet 244 to aft engine 200. Accordingto the illustrated embodiment, upper inflection point 314 and lowerinflection point 324 are defined in the same plane between leading edge64 of vertical stabilizer 32 and trailing edge 66 of vertical stabilizer32. For example, upper inflection point 314 and lower inflection point324 may be defined at a halfway point between leading edge 64 andtrailing edge 66 of vertical stabilizer 32. However, it should beappreciated that upper inflection point 314 and lower inflection point324 may be defined at any suitable location on fuselage 20. For example,upper inflection point 314 and lower inflection point 324 may be definedin a plane perpendicular to the longitudinal direction 12 that isthree-quarters of the way along junction line 68 from leading edge 64 totrailing edge 66. In addition, upper inflection point 314 and lowerinflection point 324 may be positioned at different locations along thelongitudinal direction 12 (i.e., may be in different vertical planes).It should also be appreciated that the locations of upper inflectionpoint 314 and lower inflection point 324 discussed herein are used onlyfor explaining aspects of the present subject matter. Other locationsand configurations of top surface 302 and bottom surface 304 of fuselage20 are possible and within the scope of the present subject matter.

Referring still to FIG. 5, according to an exemplary embodiment, topsurface 302 of fuselage 20 defines a tangent line 350 that extendsparallel to top surface 302 and intersects a forward lip 246 of nacelle224. According to the illustrated embodiment, tangent line 350 isdefined where Line X-X intersects fuselage 20 (approximately halfwaybetween leading edge 64 and trailing edge 66 of vertical stabilizer 32).However, other locations are possible. Notably, according to theillustrated embodiment, top surface 302 of fuselage 20 further defines arecessed portion 354 located at the aft end 16 just upstream of aftengine 200. Recessed portion 354 is defined where top surface 302 isindented inwardly toward fuselage 20 (i.e., towards the mean line 18 ofthe aircraft 10). However, because relatively higher velocity boundarylayer air cannot track recessed portion 354 as easily as lower velocityair, the relatively higher velocity air continues along a trajectorydefined by tangent line 350, thereby avoiding ingestion by aft engine200.

It should be appreciated that the shape of fuselage 20 illustrated inFIG. 5 is only one exemplary fuselage 20 shape. According to alternativeembodiments, fuselage 20 may be shaped in any manner suitable foroptimizing the ingestion of boundary layer air. For example, referringto FIG. 6, fuselage 20 may define several regions along the aft end offuselage 20, each region being concave, convex, or straight and havingvarying radii of curvature.

More specifically, according to the illustrated exemplary embodiment,fuselage 20 defines a first region 360 that extends along junction line68 between leading edge 64 and a first point along junction line 68.First region 360 is convex, e.g., when viewed looking down onto topsurface 302 from outside of fuselage 20. In addition, first region 360may have a relatively large radius of curvature, i.e., first radius 361.According to an exemplary embodiment, first region 360 may furtherdefine an average angle along its length that is approximately tendegrees or less relative to longitudinal direction 12.

Fuselage 20 also defines a second region 362 that extends along junctionline 68 between first region 360 and a second point along junction line68. Second region 362 is also convex, e.g., when viewed looking downonto top surface 302 from outside of fuselage 20. Second region 362 mayhave a radius of curvature, i.e., second radius 363, which is relativelysmall compared to first radius 361. For example, according to oneexemplary embodiment, the ratio of first radius 361 to second radius 363may be 2:1, 3:1, 4:1, or greater. Furthermore, according to an exemplaryembodiment, second region 362 may further define an average angle alongits length that is approximately twenty degrees or less relative tolongitudinal direction 12.

Fuselage 20 also defines a third region 364 that extends along junctionline 68 from second region 362 towards end of junction line 68. Forexample, third region 364 may terminate at the end of junction line 68,or at any other location forward of fan 128. Third region 364 is concaveand may have a radius of curvature, i.e., third radius 365, which isrelatively large compared to second radius 363. For example, thirdradius 365 may be approximately the same as first radius 361. It shouldbe appreciated that the regions described above are only used for thepurpose of explaining aspects of the present subject matter. There maybe fewer or more than three distinct regions, and each may be concave,convex, or have any suitable radius of curvature.

Now referring to FIGS. 7 and 8, two alternative cross-sectional views offuselage 20 will be described according to exemplary embodiments of thepresent subject matter. Although the profiles used to describe the crosssections of fuselage are different, similar reference numerals will beused to describe them. It should also be appreciated that the crosssections discussed herein are used only for explaining aspects of thepresent subject matter and are not intended to be limiting in scope. Thecross sectional profiles of fuselage 20 may vary along the length offuselage 20 as desired depending on the particular application toimprove the ingestion of boundary layer airflow into aft engine 200.

Referring now specifically to FIG. 7, a first cross section 330 will bedescribed according to an exemplary embodiment of the present subjectmatter. According to the illustrated embodiment, cross section 330 maybe taken along Line X-X of FIG. 5. However, it should be appreciatedthat cross section 330 may be located at any suitable location offuselage 20 along the longitudinal direction 12. For example, crosssection 330 may be defined at a halfway point between leading edge 64and trailing edge 66 of vertical stabilizer 32. Alternatively, crosssection 330 may be defined at a location along the longitudinaldirection 12 that is three-quarters of the way along junction line 68from leading edge 64 to trailing edge 66.

As illustrated, cross section 330 defines a horizontal reference line332 that extends along the lateral direction L between the sides ofcross section 330. In addition, horizontal reference line 332 extendsthrough the central axis 220 of the aft engine 200 (see also FIG. 4). Inthis manner, horizontal reference line 332 defines a top half 334 ofcross section 330 positioned above horizontal reference line 332 alongthe vertical direction V. In addition, horizontal reference line 332defines a bottom half 336 of cross section 330 positioned belowhorizontal reference line 332 along the vertical direction V. Notably,according to the illustrated embodiment, a top half cross sectional areaof top half 334 is greater than a bottom half cross sectional area ofbottom half 336. In this manner, inlet 244 may be configured to capturea sufficient and uniform amount of the boundary layer air flowing overfuselage 20. For example, according to an exemplary embodiment, the tophalf cross sectional area of top half 334 may be at least about tenpercent greater than the bottom half cross sectional area of bottom half336.

As also illustrated in FIG. 7, fuselage 20 further defines a referencecircle 338. Reference circle 338 is defined in the same plane as crosssection 330, has a center point 340 that corresponds with central axis220, and has a diameter equivalent to a length of horizontal referenceline 332 or slightly longer (e.g., less than twenty percent longer) thana length of horizontal reference line 332. Cross section 330 defines acircumference 342 and reference circle 338 defines a circumference 344.According to the illustrated embodiment, at least a portion of thecircumference 342 of top half 334 of cross section 330 is locatedoutside reference circle 338. In addition, at least a portion of thecircumference 342 of bottom half 336 of cross section 330 is locatedinside reference circle 338. In this manner, cross section 330 maygenerally be thicker or have a larger cross sectional area on top half334 relative to bottom half 336. Cross section 330 may be designed todisplace the boundary layer airflow to maximize the ingestion of lowvelocity air by the aft engine 200 and improve the propulsive efficiencyof aircraft 10.

As illustrated in FIGS. 7 and 8, cross section 330 may be displaced fromreference circle 338, such that fuselage 20 has an improved profile forboundary layer ingestion. The cross sectional profile may be similar forcross sections taken at other locations along the longitudinal direction12 or may vary depending on the application. However, according to anexemplary embodiment, circumference 342 of cross section 330 isequivalent to circumference 344 of reference circle 338. In this manner,the surface drag along a fuselage shaped as cross section 330 may besubstantially similar to the surface drag along a fuselage shaped asreference circle 338.

According to an alternative embodiment, horizontal reference line 332extends across a widest portion of cross section 330 along the lateraldirection L. In such an embodiment, horizontal reference line 332 may ormay not intersect central axis 220. For example, as illustrated in FIG.7, horizontal reference line 332 is positioned above central axis 220along the vertical direction V. However, the circumference 342 of crosssection 330 is once again constant. Such a configuration may be used toprovide a more uniform flow distribution on boundary layer airflowcircumferentially around fan inlet 244.

Referring again to FIG. 7, top half 334 of cross section 330 may have amaximum displacement 370 relative to reference circle 338. According tothe illustrated embodiment, the point of maximum displacement 370 of tophalf 334 of cross section 330 is at approximately 45 degrees and 315degrees about the circumferential direction relative to the verticaldirection V. Similarly, bottom half 336 of cross section 330 may have amaximum displacement 372 relative to reference circle 338. According tothe illustrated embodiment, the point of maximum displacement 372 ofbottom half 336 of cross section 330 is at approximately 135 degrees and225 degrees about the circumferential direction C₂ relative to thevertical direction V. It should be appreciated that these angles ofmaximum displacement are only approximates and may vary depending on theapplication. According to the illustrated embodiment, the maximumdisplacement 370 of top half 334 is equivalent to the maximumdisplacement 372 of bottom half 336.

As illustrated in FIG. 8, according to an exemplary embodiment, bottomhalf 336 of cross section may be tapered inward relative to referencecircle 338. More specifically, as illustrated, each side of bottom half336 is tapered along a substantially straight line between the seveno'clock and the nine o'clock positions along the circumferentialdirection C, relative to a vertical reference line (not shown). However,according to alternative embodiments, bottom half 336 may take any shapesuitable for improving the amount of boundary layer air to enter aftengine 200.

An aircraft having a fuselage shaped in the manner described aboveand/or an aft engine configured in the manner described above may allowfor capturing an optimal amount and distribution of a flow of boundarylayer air from the fuselage. More specifically, the shaping of fuselage20 results in a more uniform distribution of boundary layer airflowalong the circumferential direction C₂ of the fuselage 20 and fan inlet244. The velocity of the boundary layer air flowing into the aft engine200 may be similar from top half 334 to bottom half 336, thus improvingpropulsive efficiency while reducing vibration, noise, and wear on theplurality of fan blades 228.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. An aircraft defining a longitudinal direction, avertical direction, and a lateral direction, the aircraft comprising: afuselage extending between a forward end and an aft end along thelongitudinal direction; and a boundary layer ingestion fan mounted tothe fuselage at the aft end of the fuselage, the boundary layeringestion fan defining a centerline and comprising a plurality of fanblades rotatable about the centerline and a nacelle surrounding theplurality of fan blades, wherein the fuselage defines a cross sectionupstream of the boundary layer ingestion fan, the cross section defininga horizontal reference line extending through the centerline of theboundary layer ingestion fan to define a top half having a top halfcross sectional area and a bottom half having a bottom half crosssectional area, wherein the top half cross sectional area of the crosssection is greater than the bottom half cross sectional area of thecross section.
 2. The aircraft of claim 1, further comprising: astabilizer attached to the fuselage and extending between a leading edgeand a trailing edge, and wherein the cross section is defined betweenthe leading edge of the stabilizer and the trailing edge of thestabilizer.
 3. The aircraft of claim 2, wherein the cross section isdefined at a halfway point between the leading edge of the stabilizerand the trailing edge of the stabilizer.
 4. The aircraft of claim 2,wherein the stabilizer is a vertical stabilizer.
 5. The aircraft ofclaim 1, wherein the cross section of the fuselage defines acircumference, wherein the fuselage defines a reference circle at thecross section of the fuselage, wherein the reference circle defines acenterpoint at the centerline of the boundary layer ingestion fan,wherein at least a portion of the circumference of the top half of thecross section of the fuselage is located outside the reference circle,and wherein at least a portion of the circumference of the bottom halfof the cross section of the fuselage is located inside the referencecircle.
 6. The aircraft of claim 5, wherein a circumference of thereference circle is substantially the same as the circumference of thecross section.
 7. The aircraft of claim 1, wherein a maximumdisplacement of the top half of the cross section relative to thereference circle is equivalent to a maximum displacement of the bottomhalf of the cross section relative to the reference circle.
 8. Theaircraft of claim 7, wherein the maximum displacement of the top half ofthe cross section relative to the reference circle and the maximumdisplacement of the bottom half of the cross section relative to thereference circle are positioned at approximately 45 degrees and 135degrees from the vertical direction, respectively.
 9. The aircraft ofclaim 1, wherein a top surface of the fuselage defines a tangent line atthe cross section, the tangent line intersecting a forward lip of thenacelle.
 10. The aircraft of claim 2, wherein a top surface of thefuselage defines: a first point located in a plane perpendicular to thelongitudinal direction, wherein the first point is positioned at or aftof where the leading edge of the stabilizer meets the fuselage; a secondpoint located in a plane perpendicular to the longitudinal direction andpositioned at or forward of where the trailing edge of the stabilizermeets the fuselage; an upper midpoint; a first portion of the topsurface, the first portion extending between the first point and theupper midpoint, the first portion being convex and having a first radiusof curvature; and a second portion of the top surface, the secondportion extending between the upper midpoint and the second point, thesecond portion being convex and having a second radius of curvature, thesecond radius of curvature being smaller than the first radius ofcurvature.
 11. The aircraft of claim 10, wherein the upper midpoint islocated on the top surface of the fuselage approximately at a halfwaypoint between the leading edge of the stabilizer and the trailing edgeof the stabilizer.
 12. The aircraft of claim 2, wherein a bottom surfaceof the fuselage defines: a first point located in a plane perpendicularto the longitudinal direction, wherein the first point is positioned ator aft of where the leading edge of the stabilizer meets the fuselage; asecond point located in a plane perpendicular to the longitudinaldirection and positioned at or forward of where the trailing edge of thestabilizer meets the fuselage: a lower midpoint; a first portion of thebottom surface, the first portion extending between the first point andthe lower midpoint, the first portion being convex and having a firstradius of curvature; and a second portion of the bottom surface, thesecond portion extending between the lower midpoint and the secondpoint, the second portion being convex and having a second radius ofcurvature, the second radius of curvature being smaller than the firstradius of curvature.
 13. The aircraft of claim 12, wherein the lowermidpoint is located on the bottom surface of the fuselage approximatelyat a halfway point between the leading edge of the stabilizer and thetrailing edge of the stabilizer.
 14. An aircraft defining a longitudinaldirection, a vertical direction, and a lateral direction, the aircraftcomprising: a fuselage extending between a forward end and an aft endalong the longitudinal direction, the fuselage defining a top surfaceand a bottom surface; and a boundary layer ingestion fan mounted to thefuselage at the aft end of the fuselage, the boundary layer ingestionfan defining a centerline and comprising a plurality of fan bladesrotatable about the centerline and a nacelle surrounding the pluralityof fan blades, wherein the fuselage defines a cross section upstream ofthe boundary layer ingestion fan, the cross section defining acircumference and a horizontal reference line, wherein the horizontalreference line extends across a widest portion of the cross sectionalong the lateral direction, the fuselage further defining a referencecircle at the cross section and having the horizontal reference line asa diameter of the reference circle, wherein at least a portion of thecircumference at a top half of the cross section of the fuselage islocated outside the reference circle, and wherein at least a portion ofthe circumference at a bottom half of the cross section of the fuselageis located inside the reference circle.
 15. The aircraft of claim 14,further comprising: a stabilizer attached to the fuselage and extendingbetween a leading edge and a trailing edge, and wherein the crosssection is defined between the leading edge of the stabilizer and thetrailing edge of the stabilizer.
 16. The aircraft of claim 14, wherein acircumference of the reference circle is substantially the same as thecircumference of the cross section.
 17. The aircraft of claim 14,wherein a top surface of the fuselage defines a tangent line at thecross section, the tangent line intersecting a forward lip of thenacelle.